Stored program controlled switching systems have traditionally included a central computer that controlled switching functions in response to a program stored in memory. Although recent switching systems have distributed the call processing function among a number of system control units, many of the time-consuming tasks involved in call setup are still typically performed by a central control. For example, in one known digital switching system, the switching function is distributed among a plurality of switching modules. Each switching module has a plurality of ports and provides connections among the lines and trunks connected to the ports of that module. Calls involving lines or trunks connected to different modules are completed through a time-multiplexed switch that interconnects the modules. Each switching module includes a control unit that controls the switching function of that module. The system also includes a central control that controls the switching function of the time-multiplexed switch. Call handling in such systems requires the execution of a number of functions in addition to establishing connections. Although many of the real-time intensive tasks associated with calls, e.g., signal processing, are performed by the switching module control units, others, notably the determination of the identity of the terminating port of the switching system for each call and the path hunting required to select an available path through the time-multiplexed switch, are performed by the system central control.
One of the important advantages of a modular system of this type is that its capacity can be closely matched to the requirements of specific applications. However, as the system becomes larger and the number of switching modules increases, the performance of per-call tasks by the system central control imposes an upper limit on the overall system call processing capacity. Even when the terminating port determination function is distributed to the switching module control units, the performance of the path hunt function by the system central control undesirably limits system call processing capacity and still requires the involvement of the system central control in setting up individual telephone calls. Accordingly, the computer used as the system central control must be very reliable since a failure of that computer results in a loss of service to all the system customers. Of course such computer reliability requirements typically translate into a higher overall system cost, as for example, when duplicated, high reliability central computers are provided.
In view of the foregoing, two recognized problems in distributed control switching systems are the limitation on the call processing capacity and the high reliability requirements imposed on the central control computers, when per-call tasks such as path hunting are performed by a system central control.